At the first presidential debate Wednesday night in Denver, GOP challenger Mitt Romney aggressively rebutted President Barack Obama's charges that he would cut taxes for the wealthy and vowed not to pass any tax cuts that add to the deficit.

The wonky debate delved deeply into taxes, health care and the role of government, but it generally seemed devoid of major breakout moments.

Facing a deficit in battleground state polls, Romney set the tone early on in the debate and tried to redefine his image with voters, given that polls suggest his taped remarks dismissing the "47 percent" of Americans who don't pay income taxes and the Obama campaign's attacks on his tax plans have hurt his image, particularly on the question of whether he would help the wealthy at the expense of the middle class.

"Cut taxes for the rich? That's not what I'm going to do," Romney said. He said he would not reduce the share of taxes paid for by the wealthy.

And, he added, "I will not under any circumstances raise taxes on middle-income families."

And Romney still hasn't detailed exactly how his tax plans - including eliminating the estate tax, cutting tax rates 20 percent and squeezing deductions - could add up without hurting the middle class, helping the wealthy or increasing the deficit, a point Obama repeatedly made. After Romney talked vaguely about replacing Obamacare and Wall Street reform, Obama noted that on each of these items, Romney has been keeping his details a secret.

"He says that he's going to close deductions and loopholes for his tax plan. That's how it's going to be paid for, but we don't know the details. He says that he's going to replace Dodd-Frank, Wall Street reform, but we don't know exactly which ones. He won't tell us. He now says he's going to replace Obamacare and ensure that all the good things that are in it are going to be in there and you don't have to worry," Obama said.

"And at some point, I think the American people have to ask themselves, is the reason that Gov. Romney is keeping all these plans to replace secret because they're too good? Is it - is it because that somehow middle-class families are going to benefit too much from them?"

But Romney throughout the debate seemed more energized than the president, who seemed to stumble more than usual over his answers. Romney had one of the more memorable lines of the night when he vowed to cut funding for PBS and Sesame Street's iconic Big Bird when asked by moderator Jim Lehrer how he would handle the deficit.

"I'm sorry, Jim, I'm going to stop the subsidy to PBS," Romney said. "I'm going to stop other things. I like PBS, I love Big Bird. Actually, I like you, too. But I'm not going to - I'm not going to keep on spending money on things to borrow money from China to pay for. That's No. 1."

And Romney repeatedly sought to find the middle ground on health care, regulations and the like. He contended that his national health care plan to replace Obamacare would cover people with pre-existing conditions - but he hasn't detailed how beyond what current law allows. He defended his Massachusetts health care overhaul that Obama used as the template for his health care law, but he said that should be a model for states on a state level and not imposed at the federal level.

And he repeatedly hammered Obama on the $716 billion in cuts to Medicare providers in that law. Obama did point out that those cuts do not affect beneficiaries, but payments to insurance companies.

Romney dug in when it came to taxes, saying he did not favor raising any taxes to reduce the deficit. That would hurt growth and jobs, Romney said.

"I don't want to kill jobs in this environment," he said, while accusing the economy under Obama of imposing an "economy tax" on people.

Obama, however, repeatedly criticized Romney for failing to provide specifics.

"He's been asked over 100 times" how he would pay for his tax plan by identifying which loopholes he would eliminate. "He hasn't been able to identify them," Obama said.

And Obama sought to defend his signature health care law - telling Romney at one point, "I like it" when people refer to it as "Obamacare." The president said his policies would make sure insurance companies can't "jerk you around" and would give millions of people with pre-existing conditions access to affordable health insurance.

And he said the Medicare plan proposed by Romney running mate Rep. Paul Ryan (R-Wis.) would ultimately hurt existing seniors as well. That plan would provide vouchers to future Medicare beneficiaries to buy insurance from private companies.

"Over time what will happen is the traditional Medicare system will collapse," Obama said.